Metre
The metre (Commonwealth English http://en.wikipedia.org/wiki/Commonwealth_English) or meter (American English http://en.wikipedia.org/wiki/American_English) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light http://en.wikipedia.org/wiki/Light in absolute vacuum during a time http://en.wikipedia.org/wiki/Time interval of 1/299,792,458 of a second http://en.wikipedia.org/wiki/Second. SI prefixes applied to the metre Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo-'' = 1000) and ''millimetre (one thousandth of a metre; ''milli-'' = 1 / 1 000). The metre may be used with SI prefixes as shown. Conversions 1 metre is equivalent to: * exactly 1/0.9144 yards http://en.wikipedia.org/wiki/Yard (approximately 1.0936 yards) * exactly 1/0.3048 feet http://en.wikipedia.org/wiki/Feet(approximately 3.2808 feet) * exactly 10000/254 inches http://en.wikipedia.org/wiki/Inch (approximately 39.370 inches) History See History at http://en.wikipedia.org/wiki/Metres Timeline of definition * May 8, 1790 — The National Assembly (French Revolution) http://en.wikipedia.org/wiki/National_Assembly_%28French_Revolution%29 decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second. * March 30, 1791 — The French National Assembly accepts the proposal by the French Academy of Sciences http://en.wikipedia.org/wiki/French_Academy_of_Sciences that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian (geography)http://en.wikipedia.org/wiki/Meridian_%28geography%29 along a quadrant (one-fourth the polar circumference of the earth). * 1795 — Provisional metre bar constructed of brass http://en.wikipedia.org/wiki/Brass. * December 10, 1799 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard. * September 28, 1889 — The first Conférence Générale des Poids et Mesures http://en.wikipedia.org/wiki/Conf%C3%A9rence_G%C3%A9n%C3%A9rale_des_Poids_et_Mesures defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium http://en.wikipedia.org/wiki/Iridium, measured at the melting point of ice. * October 6, 1927 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmospheric pressure http://en.wikipedia.org/wiki/Atmospheric_pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other. * October 20, 1960 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths http://en.wikipedia.org/wiki/Wavelength in vacuum of the electromagnetic radiation http://en.wikipedia.org/wiki/Electromagnetic_radiation corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom. * October 21, 1983 — The seventeenth CGPM defines the length to be distance travelled by light http://en.wikipedia.org/wiki/Light in vacuum during a time interval of 1/299 792 458 of a second http://en.wikipedia.org/wiki/Second. See also *Metric system *SI *SI prefix *Conversion of units for comparisons with other units *Orders of magnitude (length) *Speed of light External links * History of the metre at the U.S. National Institute of Standards and Technology (NIST) * Timeline of history of the metre at the NIST * Bureau International des Poids et Measures - Lengths Category:SI units Category:International standards Category:Dimensional analysis